The scaling of analog signals has been known in the art for years. Typically, an analog signal is scaled by applying a full scale signal across a plurality of impedances connected in series. The plurality of impedances, typically resistors, are connected in series to provide a resistive divider where one end of the series impedances is coupled to ground potential while the other end receives the full scale audio signal. A scaled output is then extracted from one of a plurality of tap points located along the series impedance. By varying the tap point from which the scaled output is extracted, the magnitude of the scaled output is varied. Because of the simplicity, effectiveness, and low cost of the technique, the technique remains widely used today.
With the advent of digital circuitry, it has become common to digitally encode analog signals, such as audio signals, and to convert the digital signals back to analog signals prior to transmitting the audio signals to a user. For example, digital audio disks, otherwise known as compact discs, store audio information in a digital format on a substrate that is sensitive to the emission of light. A laser beam is bounced off the substrate such that a stream of digital data is received. The digital data is provided to a digital to analog (D/A) converter, which converts the digital data to an equivalent analog output. Typically, the analog output is reconstructed by selectively controlling the output of a plurality of current sources based upon the magnitude of the digital audio signal at a specific sampling period. Thus, an analog wave form is rebuilt using the output of a plurality of current sources. The analog wave form represents the audio signal and typically is used to provide a scaled audio signal in the manner described above.
One drawback to almost all audio signal adjust circuits is caused by the interaction between the circuitry generating the full scale audio signal and the circuitry used to generate a scaled audio signal. As previously described, signal adjust circuits typically apply an analog signal across a distributed series impedance and derive a scaled output by tapping the series impedance. Obtaining a differing scaled audio output is performed by extracting the scaled signal from a different tap point on a series impedance. However, because a DC voltage also appears across the series impedance, changing a tap point causes the DC level of the scaled output to change as well.
The change in DC level, or DC offset, of an audio signal causes speakers receiving the signal to produce an audible "pop". Over time, the DC offset will damage the sensitive coils in the speakers. Even though the DC offset is often not large enough to immediately cause physical damage to the speaker, DC offset will reduce the life span of the speakers. Because the DC offset appears as a step function, the change in DC offset, viewed from a frequency standpoint, contains many components within the frequency band of interest. Thus, the DC offset cannot be filtered from the scaled audio signal once it is produced.
Another solution to eliminate the DC offset and resulting audible "pop" is to use a series of five fixed gain operational amplifiers to provide 32 different amplitude levels, or volume settings. The operational amplifiers are intercoupled to binarily divide the received full scale audio signal in to the 32 different settings. While this technique eliminates the DC offset, it is part intensive, which adds cost and printed circuit board or integrated circuit real estate.
Thus, there exists a need in the art for circuitry that provides a scaled representation of an analog signal such that the scaled representation of the analog signal has zero DC offset without the cost of known prior art implementations.